An automotive transmission typically has multiple drive ranges with multiple ratio gearing in each range for establishing plural torque delivery paths between an engine and a torque output shaft. A driveshaft and final drive gearing deliver driving torque from the torque output shaft of the transmission to vehicle traction wheels.
An example of an automotive transmission adapted to embody the improvements of my invention is described in U.S. Pat. No. 5,305,663. That transmission includes a manual valve that can be moved between various transmission operating modes. Although my invention may be used in other transmission environments, it can be used in the transmission of the '663 patent to indicate a park position, a reverse drive mode, a neutral mode, an overdrive mode, a drive mode that omits the overdrive ratio, and a low speed drive mode.
When the transmission is conditioned for operation in the overdrive mode, any one of four forward driving ratios is available. Another forward drive mode would make available three forward driving ratios in which the highest ratio is a direct-drive. Still another drive mode would condition the transmission for operation in an underdrive mode, such as first gear ratio, with no opportunity for upshifts to higher gear ratios. The drive modes can be selected by the vehicle operator by appropriately adjusting a manual valve in a hydraulic valve assembly.
A microprocessor is used to control the operation of shift solenoid valves and pressure control solenoid valves. The microprocessor requires inputs from various engine and transmission sensors, as well as driver input information, in order to establish the appropriate functions of the transmission control system for any selected drive mode.
A manual drive range selector lever is mechanically connected to the manual valve. The position of the manual range selector lever must be read and that information must be delivered to the microprocessor so that the microprocessor can establish the proper shift schedules and the correct control pressure for any given set of operating conditions. The microprocessor also develops control functions for actuating and releasing a torque converter bypass clutch situated between the engine and the gearing of the transmission.
It is known design practice to use a transmission range selector switch for indicating the position of a driver-controlled drive range selector lever. The switch includes a first switch member having multiple contact points and a movable wiper element that engages the contact points on the switch assembly. Such a range selector position indicator switch may provide a false lever position if the switch contacts should become contaminated or if the relatively movable switch contacts or wiper elements become worn. In either case, an unreliable selector lever position signal might be made available to the microprocessor.